Among internal combustion engines (hereinafter referred to as engine) used in vehicles or the like, attention has been paid to a compression ignition gasoline engine (hereinafter referred to as a compression ignition engine) as an engine that improves both fuel consumption performance as well as exhaust performance. This compression ignition engine is designed to compress an air-fuel mixture to cause auto ignition combustion (hereinafter referred to as HCCI combustion). The compression ignition engine reduces fuel consumption because of high efficiency by high compression and of decrease in pump loss and in cooling loss by rapid combustion, as compared with spark ignition combustion which ignites and burns the air-fuel mixture by sparks of an ignition plug. Also, the compression ignition engine decreases a mass of NOx emission because of low-temperature combustion of the air-fuel mixture. This can achieve improvement both in fuel consumption performance and exhaust performance.
Means for performing the HCCI combustion include intake overheating, high compression, and introduction of an internal EGR. However, taking into consideration cost and driving by the spark ignition combustion, the introduction of the internal EGR by manipulation of valve timing is the most practicable means. In performing the HCCI combustion by the introduction of the internal EGR, a great amount of the internal EGR gas is required in a combustion chamber. This restricts new gas, and hence in a naturally-aspirated engine, the HCCI combustion can be carried out at low loads and low revolutions as shown in FIG. 1. Thus, when the compression ignition engine is applied to a vehicle, the compression ignition engine performs both the spark ignition combustion and the HCCI combustion, and achieves an engine torque requested by a driver by switching between these combustion modes.
The related art associated with combustion mode switching between the spark ignition combustion and the HCCI combustion is disclosed in, for example, JP-A-2002-130006, and JP-A-2003-106184. In the related art, when intending to perform the combustion mode switching from the spark ignition combustion to the HCCI combustion, engine parameters are changed to predetermined set values for performing the respective combustion modes, thereby to perform the combustion mode switching. Particularly, in order to achieve the HCCI combustion, a negative valve overlap period in which an exhaust valve and an intake valve are both closed in an exhaust stroke is lengthened in the HCCI combustion, as compared with the spark ignition combustion, so as to introduce a great amount of the internal EGR gas. This increases the temperature in a cylinder, thereby improving ignitability of the air-fuel mixture.
[Patent Document 1] JP-A-2002-130006
[Patent Document 2] JP-A-2002-106184